Air Brake System
General Information
A dual air brake system consists of two independent
air systems that use a single set of brake controls.
Each system has its own reservoirs, plumbing, and
brake chambers. The primary system operates the
service brakes on the rear axle; the secondary sys-
tem operates the service brakes on the front axle.
Service brake signals from both systems are sent to
the trailer.
WARNING
Do not operate the vehicle with the front brakes
backed off or disconnected. Backing off or dis-
connecting the front brakes will not improve ve-
hicle handling and may lead to loss of vehicle
control, resulting in property damage or personal
injury.
IMPORTANT: On TRACTORS, loss of air pres-
sure in the primary system causes the rear ser-
vice brakes to become inoperative; front brakes
will continue to be operated by secondary sys-
tem air pressure. On TRUCKS, loss of air pres-
sure in the primary system causes the rear
spring brakes to get modulated, controlled by a
secondary air signal. In addition, on TRACTORS
and TRUCKS, trailer brakes will be operated by
the secondary system.
Before driving your vehicle, allow time for the air
compressor to build up a minimum of 100 psi (689
kPa) pressure in both the primary and secondary
systems. Monitor the air pressure system by observ-
ing the dual system air pressure gauge and the low-
air-pressure warning light and buzzer. The warning
light and buzzer shut off when air pressure in both
systems reaches 64 to 76 psi (441 to 524 kPa).
The warning light and buzzer come on if air pressure
drops below 64 to 76 psi (441 to 524 kPa) in either
system. If this happens, check the dual system air
pressure gauge to determine which system has low
air pressure. Although the vehicle’s speed can be
reduced using the foot brake control pedal, either the
front or rear service brakes will not be operating at
full capacity, causing a longer stopping distance.
Bring the vehicle to a safe stop, and have the air
system repaired before continuing.
IMPORTANT: In the event of a total loss of ser-
vice brakes with full system air pressure, use
the parking brake control valve (yellow knob) to
bring the vehicle to a complete stop in the saf-
est location possible.
Before the vehicle can be moved, the spring parking
brakes must be released by applying an external air
source at the gladhands, or by manually caging the
parking brake springs.
WARNING
Do not release the spring parking brakes and
then drive the vehicle. There would be no means
of stopping the vehicle, which could result in se-
rious personal injury or vehicle damage. Before
releasing the spring parking brakes, make the
connection to a towing vehicle or chock the tires.
After correcting the brake system problem, uncage
the spring parking brakes before resuming normal
vehicle operation.
Brake System Operation
IMPORTANT: Before driving the vehicle, secure all
loose items in the cab so that they will not fly forward
during a full brake application. Make sure all passen-
gers are wearing seat belts.
During normal brake stops, depress the foot brake
control pedal until braking action slows down the ve-
hicle. Increase or decrease the pressure on the
pedal so that the vehicle comes to a smooth, safe
stop. Apply the spring parking brakes if the vehicle is
to be parked.
IMPORTANT: An air brake proportioning system
is used in tractor air brake systems when the
vehicle is not equipped with ABS. When operat-
ing in bobtail mode, the rear brake chambers
(whose axle load has been greatly reduced) re-
ceive reduced or proportional air pressure, while
the front axle brake chambers receive full (nor-
mal) air pressure. This results in a different
brake pedal "feel," as the pedal seems to re-
quire more travel and/or effort to slow or stop
the vehicle. However, the air brake proportioning
system actually improves vehicle control when
the tractor is in the bobtail mode of operation.
When the tractor is towing a trailer, the rear
Brake Systems
12.1
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